Materials Science · Cross-listed from physics.app-ph
Capacitance of Undoped Thin-Film Diodes
Oskar J. Sandberg, Mathias Nyman, Stefan Zeiske et al. (+3 more)
The capacitance of thin-film diodes based on undoped
semiconductors are dominated by injected charge carriers rather than doping-induced carriers. However, an analytical framework of the capacitance in these devices has remained elusive. Here, we derive an analytical description of the capacitance, fully accounting for injected charge carrier and electrode charge effects. Based on these findings, a method to extract the built-in voltage in these devices is presented. The theoretical framework is substantiated numerically by drift-
diffusion simulations and experimentally on organic solar
cells.
Read the full abstract ▾
All authors Oskar J. Sandberg, Mathias Nyman, Stefan Zeiske, Staffan Dahlstrom, Johannes Benduhn, Ardalan Armin
Materials Science · Cross-listed from cond-mat.mes-hall
Nonlinear Topological Orbital Responses of Antiferromagnetic Skyrmions
Amir N. Zarezad, Arne Brataas and Alireza Qaiumzadeh
Antiferromagnetic skyrmions evade the skyrmion Hall effect, but compensation suppresses their conventional topological charge Hall signal. We predict a semiclassical nonlinear topological orbital response of an isolated skyrmion in a $\mathcal{PT}$-symmetric hexagonal antiferromagnet without
spin-orbit coupling. In the diffusive, weak-emergent-field regime, the
spin-dependent emergent Lorentz force reshapes the carrier distribution, generating a local orbital Hall-current correction and a local orbital accumulation, both quadratic in the applied electric field. The current correction requires
spin-asymmetric longitudinal scattering, whereas the accumulation survives
spin-symmetric scattering. When the
spin-
diffusion length greatly exceeds the strip width, the current correction persists while the accumulation approaches zero. Both signals are even under electric-field reversal, odd under reversal of the skyrmion topological charge $Q$, and helicity independent within this model. These
symmetries enable rectified detection and distinguish this mechanism from the $Q$-even
quantum-regime topological orbital Hall effect.
Read the full abstract ▾
Materials Science · Cross-listed from cond-mat.str-el
Vertical quantum confinement in bulk MoS$_2$
Jairo Obando-Guevara, Álvaro González-García, Marcin Rosmus et al. (+5 more)
We experimentally observe
quantum confinement states in bulk MoS$_2$ using Angle-Resolved Photoemission Spectroscopy (ARPES). The band structure at the $\overline{\Gamma}$ point reveals
quantum well states (QWSs) linked to vertical
quantum confinement of the electrons, confirmed by the absence of dispersion in k$_z$ and a strong intensity modulation with
photon energy. Notably, the binding energy dependence of the QWSs vs $n$ does not follow the quadratic dependence of a two-dimensional electron gas. Instead, a linear behaviour is observed that is consistent with a parabolic-like
quantum well. This confinement arises from the mechanical exfoliation preparation method, which leads to the detachment of a multilayer stack from the underlying bulk. This is confirmed by Density Functional Theory (DFT) calculations. The
quantum confinement in bulk-like MoS$_2$ not only offers the opportunity to explore intersubband transitions to exploit optical properties but also provides a means to study fundamental
quantum phenomena in multilayer stacks of different thicknesses.
Read the full abstract ▾
All authors Jairo Obando-Guevara, Álvaro González-García, Marcin Rosmus, Natalia Olszowska, César González, Miguel Ángel González-Barrio, Antonio Tejeda, Arantzazu Mascaraque
Materials Science · Cross-listed from cond-mat.str-el
Hysteretic Coherence Collapse Across the First Order CDW Transition in 1T-TaS2
Turgut Yilmaz, Anil Rajapitamahuni, Asish K. Kundu et al. (+2 more)
The first order phase transition between the nearly commensurate (NC-CDW) and commensurate (C-CDW) charge density wave phases in 1T-TaS2 underpins its exotic electronic behavior, yet the spectroscopic
evolution of the low energy electronic structure across this transition remains crucial to understand. Using angle resolved photoemission spectroscopy (ARPES), we investigate the low temperature C-CDW phase, characterized by a flat band commonly associated with the lower Hubbard band and a distinct in-gap state located closer to the Fermi level.
Photon energy dependent measurements distinguish these two low energy features through their different spectral weight
evolution. Temperature dependent ARPES across heating and cooling cycles reveals that the in-gap state undergoes an abrupt collapse upon heating into the NC-CDW phase and re-emerges sharply upon cooling back into the C-CDW phase. This pronounced thermal hysteresis provides direct spectroscopic evidence of the first order nature of the transition. Furthermore, the disappearance and recovery of the in-gap state closely track the corresponding changes in resistivity, highlighting its intimate connection to the electronic reconstruction across the C-CDW/NC-CDW phase transition.
Read the full abstract ▾
All authors Turgut Yilmaz, Anil Rajapitamahuni, Asish K. Kundu, Menka Jain, Elio Vescovo
Materials Science
Temperature-Dependent nonlinear optics from first-principles: Second-Harmonic Generation in few-layers MoS$_2$
Anna Romani, Claudio Attaccalite and Myrta Grüning
We present a first-principles real-time approach to study non-linear response of solids at finite temperature. Finite temperature effects are included as renormalization of the quasiparticle energies and a dephasing term proportional to the quasiparticle lifetimes. We evaluate electron-
phonon matrix elements from Density-Functional Perturbation Theory for
lattice dynamics and then calculate quasiparticles renormalization and lifetime from the Fan and Debye-Waller terms of the electron self-energy. Electron excitations are treated at the independent particle level of approximation. We apply the approach to the second-harmonic generation (SHG) in monolayer and trilayer MoS$_2$. We observe a nontrivial temperature-dependence of the SHG due to a strong
crystal-momentum dependent quasiparticle renormalization. From the
phonon-mode analysis we find that the coupling with acoustic and shear modes determines the overall
crystal-momentum dependence respectively in monolayer and trilayer MoS$_2$. The nontrivial temperature-dependence of the SHG can help rationalize the increase of SHG intensity with increasing temperature observed in monolayer MoS$_2$ at a given
laser energy [Adv. Optical Mater. 8, 2000441 (2020)].
Read the full abstract ▾
Materials Science
High-Field Electron Transport in AlGaN alloys: A Full-Band Monte Carlo Study Based on Ab Initio Supercell Calculations
Animesh Datta, Matinehsadat Hosseinigheidari and Uttam Singisetti
AlGaN
alloys are promising wide and ultra-wide-bandgap
semiconductors for next-generation power and RF electronics applications. To realize the full potential of AlGaN based devices, it is important to understand the electron transport to accurately predict device performance and identify material limits under various operating conditions. In this work, the high-field electron transport properties of AlxGa1-xN are investigated using a supercell based full band Monte Carlo method. The supercell approach is employed to explicitly capture the true disorder of the
alloy system, enabling a more realistic description of carrier transport. The velocity field characteristics are calculated across a range of Al compositions to evaluate key transport metrics, including peak velocity, saturation velocity, and critical electric field. The role of different scattering mechanisms is studied in detail to understand the high field transport mechanism in the AlGaN
alloy system. In addition to steady state transport, transient electron dynamics are examined for various Al fractions to study velocity-overshoot behavior, which is especially important for improving the performance of scaled RF devices. Finally, the temperature dependence of the velocity field characteristics in ultra-wide-bandgap Al0.75Ga0.25N is investigated to assess its transport performance under high temperature conditions. These results provide a detailed understanding of high-field transport in AlGaN
alloys and offer guidance for the design of AlGaN-based RF and power electronic devices.
Read the full abstract ▾
Materials Science
Exciton Coherence in CsPbBr3 Nanocrystals is Bounded by Phonon-Mediated Bright-Triplet Relaxation
Tara Šverko, Annette J. Jones, Chantalle J. Krajewska et al. (+6 more)
Scalable sources of indistinguishable single
photons or entangled
photon pairs are fundamental to many
quantum photonic technologies. Colloidal lead halide perovskite nanocrystals are promising such sources, but their
exciton coherent properties are not fully understood. We show that in single CsPbBr3 nanocrystals at 4 K, acoustic
phonon-mediated
exciton fine structure relaxation (EFSR) drives leakage of population between the bright
exciton triplet states, competing with the radiative lifetime. The leakage pathway reaches 0.64 $\pm$ 0.04 of the radiative rate, bounding state coherence to 0.81 times the transform limit at 4 K. This pathway is intrinsic to the nanocrystal, not its environment, making it a tractable target for materials and device design. We identify fine structure and acoustic
phonon engineering as effective intrinsic routes to higher coherence in perovskite
quantum light sources.
Read the full abstract ▾
All authors Tara Šverko, Annette J. Jones, Chantalle J. Krajewska, Peter C. Sercel, Alexander L. Efros, Alexander E. Kaplan, Niamh L. Brown, Stefano Toso, Moungi G. Bawendi
Materials Science · Cross-listed from cs.LG
Online Surrogate Repair: Decoupling High-Fidelity Feedback from Search Length in Closed-Loop Discovery
Xiaotang Feng, Philip Torr and Bruno Andreis
Closed-loop AI scientists can generate candidate designs at low marginal computational cost, whereas reliable feedback may require wet-lab synthesis, characterization, or high-fidelity computation. Addressing this imbalance through custom laboratory automation remains infrastructure-intensive and costly, while replacing new experiments with a fixed surrogate leaves persistent model errors that can be amplified by optimization. We propose online surrogate repair (OSR), a closed-loop
algorithm that uses sparse high-fidelity evaluations to update the surrogate throughout a longer
agent search conducted primarily with inexpensive surrogate feedback. An acquisition rule selects which designs from the
agent's accumulated proposals receive high-fidelity evaluation, and the resulting labels update the surrogate used in subsequent episodes. Across controlled synthetic environments, we demonstrate that improving global surrogate fit does not necessarily reduce maximum regret, whereas Q90-UCB and expected improvement (EI) substantially reduce regret by directing evaluations toward regions that determine the optimizer's decisions. On MADE, controls receiving high-fidelity feedback after every episode require $6.36$–$7.23\times$ more oracle queries to match Online EI under two LLM orchestrators and $10.27\times$ more under the non-LLM Chemeleon+MLIP workflow. Online surrogate repair introduces a novel third feedback regime between fixed-surrogate operation and high-fidelity feedback after every episode, separating the frequency of high-fidelity evaluation from the duration of the
agent's search.
Read the full abstract ▾
Materials Science · Cross-listed from quant-ph
Microwave-Free $^{13}$C Hyperpolarization of Diamond Particles Enabled by Magic Angle Spinning and NV Centers
Rémi Blinder, Guzel Musabirova, Daehee Kim et al. (+6 more)
Nuclear hyperpolarization from optically pumped color centers in solids offers an alternative to conventional microwave-driven dynamic nuclear polarization (DNP). Diamond can host the nitrogen vacancy (NV) center, whose ground
spin state can be readily polarized by light at room temperature, making diamond a candidate platform for nuclear hyperpolarization. We report $^{13}{\rm C}$ nuclear hyperpolarization in randomly oriented diamond particles with sizes ranging from 0.2 to 2 $\mu$m, both at natural $^{13}{\rm C}$ abundance (1.1 %) and at 20 % isotopic enrichment, at
magnetic fields of 7.1 T and 9.4 T. The protocol combines optical illumination with magic angle spinning (MAS) and does not require microwave irradiation. By investigating the nuclear polarization as a function of the MAS frequency between 0 and 6 kHz at the
magnetic field of 7.1 T, we find maximum light-induced polarization enhancements of $280$-fold for the isotopically enriched sample and $411$-fold for the natural abundance sample. Under continuous illumination, steady-state absolute $^{13}{\rm C}$ polarization levels above 0.1 % are reached. A model involving optical pumping of NV centers and
spin dynamics near level anticrossings (LACs) in three-
spin clusters formed by NV, a substitutional nitrogen (P1) and $^{13}{\rm C}$ is used to describe these findings. The protocol strongly mitigates the effect of the anisotropy of the NV
spin Hamiltonian, allowing more than $99.9\%$ of NV orientations to participate in the polarization transfer process. These results represent a first step toward transferring nuclear polarization from diamond particles to external
nuclei, with potential applications in sensitive and high-resolution NMR at room temperature.
Read the full abstract ▾
All authors Rémi Blinder, Guzel Musabirova, Daehee Kim, Anshuman Nayak, Jason Twamley, Viateschlav N. Agafonov, Raiker Witter, Jörg Matysik, Fedor Jelezko
Materials Science · Cross-listed from cond-mat.str-el
Raman signatures of a non-reciprocal magnetic phase transition in Ca$_2$RuO$_4$
Giacomo Jarc, Giovanni Tartaglia, Francesco Gabriele et al. (+17 more)
The
magnetic behavior of Ca$_2$RuO$_4$ represents a unique puzzle due to the interplay of strong electronic correlations, magneto-elastic interactions, and large
spin-orbit coupling. At low temperatures, an anomalous Mott insulating state emerges, characterized by a complex antiferromagnetic order with a collective amplitude excitation of the
magnetic moment which has been discussed in analogy with the
Higgs mode. We report here evidence of a
magnetic first-order phase transition driven by an out-of-plane
magnetic field along the crystallographic $c$-axis of Ca$_2$RuO$_4$. Raman measurements in
magnetic field reveal the emergence of mode of
magnetic origin and a concomitant modification of the coupling between a
phonon and the
Higgs amplitude mode. Both the Raman features are characterized by a non-reciprocal hysteresis in
magnetic field. Surprisingly, the observed phase transition does not affect the Raman scattering from the in-plane
magnons, indicating that the dipolar antiferromagnetic order is preserved. This is consistent with the onset of a new quadrupolar field-controlled state, whose fluctuations structure can trigger an hybridization between the
lattice and the
magnetic Higgs mode resulting in the observed Raman features.
Read the full abstract ▾
All authors Giacomo Jarc, Giovanni Tartaglia, Francesco Gabriele, Filomena Forte, Anita Guarino, Angela Montanaro, Enrico Maria Rigoni, Nitesh Khatiwada, Costanza Lincetto, Gabriele Bartolini, Antonio Mastropasqua, Shahla Yasmin Mathengattil, Marco Malvestuto, Muhammad Waqee Ur Rehman, Rosalba Fittipaldi, Joachim Deisenhofer, Alexander A. Tsirlin, Antonio Vecchione, Mario Cuoco, Daniele Fausti
Materials Science
Switchable Altermagnetism in a Layered van der Waals Metal-Organic Framework Driven by Spin-Crossover
Diego López-Alcalá, Alberto M. Ruiz, Andrei Shumilin et al. (+1 more)
Dynamical control of altermagnetism is a key requirement for translating its unique
spin-dependent functionalities into practical spintronic devices, yet effective switching mechanisms remain largely unexplored. Here, we open an unprecedented, versatile and programmable route based on
spin-crossover to switch altermagnetism on demand in molecular materials. Using density functional calculations, we demonstrate an altermagnetic ground state in the layered van der Waals MOFs MnX$_2$(tdz)$_2$ (X = Cl, Br; tdz = thiadiazole), stabilized by anisotropic interlayer exchange interactions, which gives rise to a characteristic d-wave momentum-space
spin splitting and an associated
spin-splitter transport response. Our findings reveal that under hydrostatic pressure, a high-
spin to low-
spin transition reconfigures the Mn d-orbital occupation, thus modifying the
magnetic exchange network, and stabilizing a different antiferromagnetic ground state whose
symmetry suppresses the nonrelativistic
spin splitting. Crucially,
spin-crossover switches altermagnetism not by directly altering the electronic structure, but by changing the
symmetry of the
magnetic ground state. These results establish molecular
spin-crossover altermagnets as a platform for externally reconfigurable spintronic devices.
Read the full abstract ▾
All authors Diego López-Alcalá, Alberto M. Ruiz, Andrei Shumilin, José J. Baldoví
Materials Science
Benchmarking Universal Machine Learning Force Fields for Crystal Structure Prediction of High-Energy Molecular Systems
Musiha Mahfuza Mukta, Osman Goni Ridwan, Romain Perriot et al. (+1 more)
Recent developments of universal machine learning interatomic potentials (UMLIPs) offer a fast route for screening molecular
crystals based on geometry relaxation and energy ranking, but their reliability across chemically diverse energetic materials remains elusive. In particular, it is unclear whether or not these UMLIPs are over-sensitive to break the desired molecular connectivity for relaxing the periodic
crystals. Herein we tested the hypothesis that classical force-field pre-relaxation can provide a more suitable starting geometry for subsequent UMLIP relaxation on a large database of high energy molecular
crystals. Three models (MACE, MACE-OFF and UMA) in conjunction with the General Amber Force Field (GAFF) were applied to test this hypothesis. Among them, direct MACE-OFF and UMA showed very high relaxation success and preserved the reference geometries most closely, but they still exhibit failures for some rare cases. Using GAFF pre-relaxation can systematically reduce the number of failed relaxations with lower computational costs. Our comparative failure and robustness analyses revealed distinct trade-offs among the evaluated models. Among them, MACE-OFF achieves a better compromise between potential energy surface smoothness, structural fidelity, and stress convergence, serving as a good choice to provide a reliable foundation for automated structural optimization.
Read the full abstract ▾
All authors Musiha Mahfuza Mukta, Osman Goni Ridwan, Romain Perriot, Qiang Zhu
Materials Science · Cross-listed from eess.SY
Noninvasive Anisotropic Identification of Magnetic Properties in Toroidal Shaped Magnetic Steel
Xusen Qin, Reza Zeinali and Mitrofan Curti
This research proposes a model-free inverse approach for identifying the nonlinear anisotropic single-valued
magnetic constitutive characteristics of soft
magnetic materials from boundary measurements. Using the harmonic approximation, the internal
magnetic flux density B and
magnetic field strength H are independently reconstructed from the measured boundary data, thus avoiding the limitation of using a predefined parametric B–H mapping function. Subsequently, the reconstructed fields are spatially averaged along selected collection lines to recover the B–H characteristics of the principal directions. Numerical results demonstrate high reconstruction accuracy for unperturbed data and good robustness against Gaussian perturbations of the boundary measurements, with the overall mean error remaining below 5% for a 10% noise level. These results indicate that under the harmonic-field assumptions, the field-based reconstruction provides a simple and computationally efficient framework for non-invasive identification of nonlinear anisotropic
magnetic constitutive characteristics.
Read the full abstract ▾
Materials Science · Cross-listed from cond-mat.mes-hall
Direct Realization of Near-Ideal Carbyne in Ultrathin Boron Nitride Nanotubes
Iryna Ivanenko, Pietro Marabotti, Yifan Zhang et al. (+8 more)
Carbyne, the sp-hybridized one-dimensional allotrope of carbon, is predicted to be the stiffest known material, with electronic and optical properties set by a single structural parameter, the bond length alternation. However, its intrinsic properties have never been measured: chains synthesized through molecular chemistry carry endgroup and finite-length perturbations that persist even in the longest molecules available, while chains grown inside carbon nanotubes strongly couple to the host, which renormalizes their vibrational frequency by up to 110 cm$^{-1}$ in a diameter-dependent manner. Here, we show that encapsulating and thermally converting hydrogen-capped polyynes inside ultrathin boron nitride nanotubes, structural analogues of carbon nanotubes but electrically insulating, yields carbyne chains in a near-ideal regime, where endgroup, finite length, and host-guest perturbations are reduced to secondary effects. Statistical Raman spectroscopy across 245 locations returns a vibrational frequency distribution an order of magnitude narrower than in carbon nanotubes, an anharmonicity consistent with the universal law for carbyne-like materials, and a bond length alternation matching correlated calculations for the free chain. No photoluminescence is detected, despite the transparent host, as expected for the dipole-forbidden emission of an unperturbed carbyne chain. Boron nitride nanotubes give experimental access to carbyne in its near-ideal form.
Read the full abstract ▾
All authors Iryna Ivanenko, Pietro Marabotti, Yifan Zhang, Getulio Silva e Souza Júnior, Johannes M. A. Lechner, Pablo Hernández López, Martin Magg, Shivani Shivaprakash, Carlo Spartaco Casari, Sebastian Heeg, Benjamin S. Flavel
Materials Science · Cross-listed from cond-mat.mes-hall
Layer-Dependent Phonons, Excitons, and Magneto-Optical Phenomena in CrSBr: A Mini Review
Muhammad Aftab, Chinmay Kumar Mohanty, Zain Ashfaq et al. (+5 more)
Two-dimensional layered
magnetic materials offer a versatile platform for exploring low-dimensional magnetism and coupled many-body interactions in these materials. Chromium sulfur bromide (CrSBr) is a promising candidate for advanced spintronic and optoelectronic applications because of its intrinsic air stability, semiconducting nature, strong in-plane anisotropy, and A-type antiferromagnetic ordering. This review summarizes recent advances in the understanding of the layer-dependent vibrational and excitonic properties of CrSBr, as well as its magneto-optical response, from bulk
crystals to the monolayer limit. We examined its
crystal structure,
magnetic anisotropy, and interlayer
spin reorientation, followed by insights into vibrational dynamics and
spin-
phonon coupling. Particular emphasis is placed on the excitonic landscape, including
magnetic-field-sensitive photoluminescence, localized excitonic states, and the coexistence of Frenkel- and Wannier-Mott
excitons in the bandgap. Finally, we discuss the challenges and prospects of harnessing the unique layer-dependent properties of CrSBr in spintronic, magneto-optical, and
quantum photonic technologies.
Read the full abstract ▾
All authors Muhammad Aftab, Chinmay Kumar Mohanty, Zain Ashfaq, Warisha Mehmood, Xiaoli Wang, Clément Faugeras, Wajid Ali, Maciej R. Molas
Materials Science
Quantifying thermal and driven magnon populations with femtosecond noise correlation spectroscopy
F. S. Herbst, M. A. Weiss, A. Leitenstorfer et al. (+5 more)
Precise knowledge of the total number of
magnons, including both coherent and incoherent (e.g. thermal) excitations, is imperative for the advancement of fundamental
spin-wave physics and the development of next-generation magnonic devices. In particular, quantifying
magnons is key to understanding
magnon transport phenomena, the nonlinear regime, or ultrafast magnetization dynamics. Typically, incoherent
magnons are accessed by frequency-domain techniques, which lack the temporal resolution required for ultrafast processes, while ultrafast time-domain methods are generally sensitive only to the coherent dynamics. In this work, we demonstrate that femtosecond noise correlation spectroscopy enables a fully quantitative, time-domain measurement of both thermal and coherently excited
magnon modes in bismuth-substituted yttrium iron garnet driven by a free-running microwave. We model the experimental data and extract the
magnon number by simulating the
magnon band structure of the sample, the magneto-optical response function, and the optical spot size used in the experiment. Our analysis establishes a connection between
magnon mode calculations and experimentally accessible
magnetic properties and fiducially reproduces the waveform and amplitude of the magneto-optical correlation signal for different experimental conditions. These results open a new pathway towards the optical
tomography of
magnon modes in non-linear or non-
equilibrium conditions and can be readily extended to study ultrafast incoherent dynamics in other condensed matter systems.
Read the full abstract ▾
All authors F. S. Herbst, M. A. Weiss, A. Leitenstorfer, M. Lammel, N. Beaulieu, J. Ben Youssef, R. Schlitz, S. T. B. Goennenwein
Materials Science
How to enhance the applicability of Murnaghan equation of state
V.V. Bannikov
The paper presents the brief overview of known isothermal equations of state for solids and of theoretical background to construct them, focusing on widely used Murnaghan equation. Since the latter has a limited range of application, failing to reproduce reliably both near-
equilibrium and high-pressure regions simultaneously, the paper proposes simple, but rather efficient way to improve it. The suggested modification favors its applicability at high pressures, allowing to expand considerably the region of reliable reproducing of experimental or computational data. The proposed improved equation has been tested in fitting of U(V) data, obtained within ab initio computations, as well as of experimental V-P data for a representative selection of solids. In comparison with ordinary Murnaghan equation, it has found significant advantages, extending the range, where the computational data may be reproduced trustworthy (from the
equilibrium state to the pressures, appreciably higher, the ordinary equation could). As for the fitting of experimental data, the proposed improved Murnaghan equation reproduces them, in the presented examples, at least not worse than other well-known equations (Vinet, Birch-Murnaghan, Mao) do. Some aspects of fitting process, as well as some alternative approaches to obtain the proposed equation of state in various forms are also discussed.
Read the full abstract ▾
Materials Science
Modular and Cost-effective Scanning Photocurrent Microscopy System for Sub-micron characterization of 2D optoelectronic devices
Nuria Jiménez-Arévalo, Dan Zheng, Yong Xie et al. (+8 more)
Scanning photocurrent microscopy (SPCM) is a powerful technique for probing local optoelectronic phenomena in 2D semiconducting devices. However, commercial setups remain costly, complex and often lack flexibility and adaptability. In this work, we present a home-built SPCM platform built around the retrofitting of a conventional metallographic microscope by coupling it with different light sources (single-mode fiber-coupled
lasers and multimode fiber-coupled high-power LEDs), a motorized XY stage, a digital camera and an electronic readout module. This system enables simultaneous acquisition of photocurrent and reflection intensity maps, requiring minimal modifications of the microscope. We reached sub-micron spatial resolution and high imaging fidelity by correlating photocurrent maps with reflection maps, optical micrographs and AFM topography data on different devices fabricated with different materials (InSe, MoS2, WSe2, Gr), on different substrates (Si/SiO2, compact disk). This work provides a reliable, accessible and reproducible high-performance SPCM platform that can be easily implemented in most laboratories for microscale optoelectronic characterization of 2D devices.
Read the full abstract ▾
All authors Nuria Jiménez-Arévalo, Dan Zheng, Yong Xie, Peng Cheng, Yi Liu, Rafael Luque Merino, Haiwen Yu, Tao Wang, Riccardo Frisenda, Andres Castellanos-Gomez, Qinghua Zhao
Materials Science
Metallic-Phase-Fe$_3$GaTe$_2$ Enabled Interface Engineering for Self-Powered and High-Gain WS$_2$ Photodetectors
Wajid Ali, Ming Huang, Juan Li et al. (+7 more)
Two-dimensional transition-metal dichalcogenides offer strong light-matter interaction but suffer from inefficient carrier separation and contact-related losses in photodetectors. Here, we demonstrate a high-gain WS$_2$/Fe$_3$GaTe$_2$ van der Waals
heterostructure photodetector, where metallic Fe$_3$GaTe$_2$ serves as an active interfacial contact. The work-function mismatch, together with interfacial charge redistribution and asymmetric contact geometry, contributes to a built-in field that supports self-powered photodetection at zero bias. Under 450 nm illumination, the device delivers a zero-bias responsivity of 23.5 A/W and an apparent external
quantum efficiency of 6.4 x 10$^3$%. At -1 V biasing, the
heterostructure exhibits photoresponse at 450, 520 and 633 nm, achieving a responsivity of 9.7 x 10$^3$ A/W and a noise derived specific detectivity of 2.3 x 10$^13$ Jones at 100 Hz under 450 nm illumination. The high photoresponse is attributed to interfacial carrier separation, efficient extraction, and a likely contribution from trap-assisted photogating in multilayer WS$_2$. These results establish Fe$_3$GaTe$_2$-enabled interface engineering as an effective route for self-powered, highly sensitive 2D photodetectors.
Read the full abstract ▾
All authors Wajid Ali, Ming Huang, Juan Li, Jianhua Huang, Liuli Yang, Sajid Ur Rehman, Chinmay K. Mohanty, Zahir Muhammad, Ziwei Li, Maciej R. Molas
Materials Science
Atomistic Origin and Strain Control of the Finite-Temperature Dielectric Response in BaTiO3
Ryotaro Sahashi, Po-Yen Chen and Teruyasu Mizoguchi
Soft-mode theory specifies a frequency, not what a local polar unit does. What real-space motion underlies the dielectric response of BaTiO3 therefore remains unclear. Using electric-field-induced molecular dynamics with a machine-learning force field, we show that the permittivity tracks the field-induced angular redistribution of local Ti-O off-centering, not its magnitude. Temperature and biaxial strain modify the local structure through different microscopic routes, yet both dielectric responses collapse onto a common relation with the same orientational descriptor, providing a real-space counterpart to soft-mode behavior.
Read the full abstract ▾
Materials Science
Groove-shaped defects in as-grown (001)-oriented $\beta$-Ga$_2$O$_3$ epilayers prepared by halide vapor phase epitaxy
Yongzhao Yao, Daiki Katsube, Hirotaka Yamaguchi et al. (+1 more)
Groove-shaped defects (GSDs) degrade the surface flatness of as-grown (001)-oriented $\beta$-Ga$_2$O$_3$ epilayers and necessitate chemical mechanical polishing before device fabrication, increasing processing costs and the risk of damage. We investigated the morphology, subsurface structure, and formation mechanism of GSDs in a homoepitaxial layer grown by halide vapor phase epitaxy using synchrotron X-ray topography and electron microscopy. The GSDs extended several millimeters along [010] and consisted predominantly of (-102) basal facets bounded by steep (100) sidewalls. Their wafer-scale distribution showed a spatial correspondence with variations in wafer
curvature, suggesting that local surface orientation influences GSD formation. Careful
alignment of surface images with transmission X-ray topographs revealed no one-to-one correspondence between GSDs and substrate defects, providing no evidence that substrate dislocations serve as their nucleation sites. Instead, transmission electron microscopy revealed planar defects localized near the terminal boundaries where the faceted GSD sectors met the surrounding (001) growth region; no such defects were observed in specimens extracted from the middle of GSDs. These defects exhibited {\alpha}-fringe contrast characteristic of inclined translational planar defects, with the dominant segments assigned to the (1-21) plane. The observations suggest that variations in local surface orientation and step supply may promote three-dimensional faceted growth, producing persistent (-102)/(100) sectors. The localized planar defects are therefore interpreted as consequences of growth-sector coalescence rather than the origins of GSD nucleation. These findings provide insight into the roles of wafer
curvature and surface step supply in GSD formation.
Read the full abstract ▾
All authors Yongzhao Yao, Daiki Katsube, Hirotaka Yamaguchi, Yukari Ishikawa
Materials Science · Cross-listed from quant-ph
Electron beam driven collective self hybridized exciton polaritons
Parsa Darman, Maximilian Black, Prabhdeep Singh et al. (+5 more)
Self-hybridized
exciton polaritons in layered
semiconductors emerge from the hybridization of excitonic resonances with confined photonic modes in films and provide a versatile platform for controlling light–matter interactions at the nanoscale. Yet, the nature of
exciton–
photon interactions and the collective and synchronous strong coupling of multiple
excitons to the same photonic mode remain largely unexplored. Here, we use complementary momentum-resolved photoluminescence and cathodoluminescence spectroscopy to compare self-hybridized
exciton polaritons in Ruddlesden–Popper perovskite flakes under optical and electron-beam excitation. We find that cathodoluminescence exhibits remarkably larger polaritonic level repulsion than photoluminescence, revealing an enhanced effective coupling strength under electron-beam excitation. We attribute this enhancement to the collective coherent excitation of multiple
excitons by fast electrons, which couple to the same photonic mode and increase the interaction strength according to a $g_N=g_1\sqrt{N}$ scaling, with an effective number of up to $N\approx23$ coherently coupled excitonic oscillators. By reducing the electron kinetic energy, we further uncover a crossover to a partially incoherent excitation regime in which higher-order Fabry–Pérot
exciton-polariton resonances disappear, leaving only the lowest polariton branches while coherent transition radiation persists. This transition arises because low-energy electrons excite
excitons over extended spatial volumes, washing out the phase coherence required to sustain collective coupling and form polaritonic resonances. Our work shows that electron beams can actively modify collective light–matter coupling in layered
semiconductors and establishes cathodoluminescence as a route to access excitation regimes beyond all-optical excitation schemes.
Read the full abstract ▾
All authors Parsa Darman, Maximilian Black, Prabhdeep Singh, Masoud Taleb, Victor DeManuel-Gonzalez, Sara Darbari, Fatemeh Chahshouri, Nahid Talebi
Materials Science
Coupled structural and electronic evolution under pressure in CuIr2Se4, CuRh2S4, and CuRh2Se4
M. Emi, M. Shiomi, K. Kojima et al. (+15 more)
Spinel chalcogenides provide a platform for investigating the interplay among metallic, superconducting, and pressure-induced insulating states. Here, we combine synchrotron powder X-ray diffraction and electrical-resistivity measurements to investigate the pressure
evolution of CuIr2Se4, CuRh2S4, and CuRh2Se4 over pressure ranges extending beyond those previously explored. High-pressure diffraction reveals closely related monoclinic supercells in all three compounds. For CuIr2Se4 and CuRh2S4, constrained profile fits based on structural models relaxed using density functional theory are compatible with Phase-IV-type bond-disproportionated structures, whereas the data for CuRh2Se4 establish a compatible monoclinic unit
cell without resolving its atomic-scale ordering pattern. Insulating-like transport develops abruptly over a narrow pressure range in CuIr2Se4 but more gradually over broader pressure ranges in the Rh-based compounds, in close correspondence with their respective structural transformations. We also establish previously unreported bulk superconductivity in CuIr2Se4 at ambient pressure: zero resistance is attained at 0.29 K, and the accompanying ac diamagnetic response is consistent with nearly complete superconducting shielding. These results establish a close relationship between the formation of the high-pressure monoclinic phases and the
evolution toward insulating transport, and demonstrate that transition-metal and chalcogen substitutions tune the characteristic pressure scales and the competition with superconductivity within a closely related structural framework.
Read the full abstract ▾
All authors M. Emi, M. Shiomi, K. Kojima, K. Sugimoto, T. Karasawa, H. Suzuki, M. Takahashi, K. Oka, H. Kadobayashi, S. Kawaguchi-Imada, N. Hirao, T. Ohashi, D. Ito, T. Kubo, M. Matsushita, M. Nohara, K. Matsubayashi, N. Katayama
Materials Science
Excitons probe intrinsic flat band Mottness in a van der Waals heterostructure
Xinyue Huang, Xintong Tan, Haowei Chen et al. (+12 more)
Excitons provide a sensitive optical probe of electronic correlations in nearby two-dimensional materials, yet their coupling to intrinsic flat-band Mott systems remains largely unexplored. Here we combine gate-tunable optical spectroscopy with first-principles calculations to study monolayer WSe$_2$ in direct contact with the van der Waals Mott
insulator Nb$_3$Cl$_8$. The gate
evolution of WSe$_2$ excitonic resonances reveals signatures of a correlation-reconstructed Mott gap in Nb$_3$Cl$_8$ that is absent from the single-particle band picture. In the electron-doped regime, the WSe$_2$ 2s Rydberg
exciton undergoes a multistage
evolution and develops into interlayer attractive and repulsive polaron branches, showing that a Rydberg
exciton can be dressed by strongly correlated flat-band electrons in an adjacent Mott layer. Under an out-of-plane
magnetic field,
spin-polarized Nb$_3$Cl$_8$ states further induce valley-selective
exciton coupling, producing a strongly enhanced circular polarization of the WSe$_2$
exciton emission. These results extend
exciton-based sensing and
exciton-polaron physics to intrinsic flat-band Mott materials, providing an optical route to probe and engineer correlation-driven interfacial quasiparticles.
Read the full abstract ▾
All authors Xinyue Huang, Xintong Tan, Haowei Chen, Yingzhou Huang, Yushen Zhou, Yuchen Gao, Zhijie Ma, Chengxin Xiao, Kenji Watanabe, Takashi Taniguchi, Jianpeng Liu, Zuxin Chen, Youguo Shi, Wang Yao, Yu Ye
Materials Science · Cross-listed from physics.class-ph
Formation of grain boundaries in ductile single crystals under plane-strain simple shear: a block-coordinate finite element method
Khanh Chau Le and Thanh Danh Nguyen
Large plastic deformation can drive an initially uniform single
crystal to spontaneously subdivide into misoriented grains separated by thin dislocation walls – a pattern-forming instability rooted in the loss of convexity of the
crystal's elastic energy at large strain. We study this phenomenon for a ductile
crystal in plane-strain simple shear within continuum dislocation theory, using a polyconvex (Ciarlet–Geymonat) elastic energy that guarantees existence of minimizers for the coupled deformation–slip problem. Minimizing over the plastic slip yields a condensed energy of double-well form whose non-quasiconvexity favours a lamellar microstructure; the gradient of the geometrically necessary dislocation density regularizes it, giving the grain boundaries a finite thickness and energy as functions of the misorientation angle. A block-coordinate finite element scheme – alternating a convex non-smooth solve for the slip with a Levenberg-regularized Newton solve for the deformation – resolves this microstructure numerically and detects its spontaneous onset, reproducing the lamellar grain structure in agreement with the closed-form analysis.
Read the full abstract ▾
Materials Science
Quantum hot carrier spectra in plasmonic catalysis
Yu Chen, Hanwen Jin, Fei Gao et al. (+2 more)
Vibrational activation of admolecules on metal nanoparticles is an elementary step in plasmonic catalysis, yet the underlying dynamics driven by hot carriers is not fully understood in the
quantum regime. Using an atomistic description of plasmonic hot carrier generation, we investigate vibrational excitation and dissociation of oxygen on silver nanoparticles as a function of diameter D. As D reduces from the classical to
quantum-sized regime, quantized distribution of hot carriers emerges with increasing population in the high-energy regions. These highly energetic hot carriers deliver more efficient vibrational coupling and dissociation. The rate of vibrational excitation shows a linear 1/D scaling, which results from Landau damping. It turns nonlinear at elevated light intensities due to vibrational heating generated by multiple electron scattering. The finding of quantized distribution of hot carrier in plasmonic catalysis opens new avenues for selective control and nonthermal energy conversion.
Read the full abstract ▾
All authors Yu Chen, Hanwen Jin, Fei Gao, Johannes Lischner, Shiwu Gao
Materials Science
Metastable polar order and phase competition in Carpy-Galy LaTaO4
Yongjin Shin
Carpy-Galy LaTaO4 undergoes a structural sequence linking the antipolar monoclinic P21/c ground state and the polar orthorhombic Cmc21 phase through an incommensurately modulated regime. First-principles calculations identify a metastable polar monoclinic P21 phase as a commensurate intermediate in the phase-transition landscape. As a common subgroup of P21/c and Cmc21, P21 provides a
symmetry-connected route between the established polymorphs. Mode-resolved energy surfaces for these
symmetry-breaking pathways show that coupling between the respective primary order parameters and an isosymmetric Gamma_1+ relaxation stabilizes the P21 minimum along both pathways. Consequently, the P21/c -> P21 transformation has a finite energy barrier, while a restricted soft-mode model retains competing P21 and Cmc21 basins over an illustrative range of harmonic stiffness.
Lattice-metric and
neutron-diffraction comparisons indicate a shared S_2+-dominated displacement character between the calculated commensurate state and the experimental IC-o modulation, supporting the use of P21 as a commensurate structural reference for IC-o. Finally, composition-dependent energetics suggest chemical routes for tuning polar-antipolar competition.
Read the full abstract ▾
Materials Science · Cross-listed from cond-mat.mes-hall
Self-Trapping Enabled Highly Bright Momentum-Indirect Interlayer Excitons
Dong Yang, Zisheng Gong, Hao Wen et al. (+6 more)
Interlayer
excitons in two dimensional material
heterostructures exhibit large
exciton binding energies and long lifetimes, making them ideal platforms for studying excitonic devices and many body
quantum phenomena. However, the spatially separated electron and hole nature of IXs reduces their oscillator strength by two orders of magnitude compared to intralayer
excitons. Achieving high efficiency IX emission remains challenging and requires optimal material selection with appropriate momentum matching and meticulous device fabrication. Here we demonstrate a highly bright momentum indirect IX emission within
heterostructures formed between 2D perovskites and monolayer transition metal dichalcogenides. The
quantum yield of IX emission reaches 35.2% on average, over 50 times higher than that of the corresponding constituent TMD monolayer, with the highest value exceeding 60%. Notably, the radiative recombination efficiency of this momentum indirect IX exceeds that of momentum direct IXs in monolayer TMD-based
heterostructures by two orders of magnitude. We suggest that the remarkably bright IX emission in our
heterostructure originates from IX self trapping, induced by strong
exciton phonon coupling arising from the soft
lattice nature of the 2D perovskite. Our findings provide new insights into achieving high IX emission efficiency and open new avenues for exploring long lifetime excitonic devices.
Read the full abstract ▾
All authors Dong Yang, Zisheng Gong, Hao Wen, Yue Hu, Kaichen Jiang, Baixu Xiang, Weibo Gao, Qihua Xiong, Dehui Li
Materials Science · Cross-listed from physics.atm-clus
Core-Level Spectroscopy Decodes Bond-Alternation Dynamics of Cyclo[18]Carbon
Minrui Wei, Zeyu Liu, Sheng-Yu Wang et al. (+4 more)
The advent of X-ray free-electron
lasers and high-harmonic generation has made time-resolved X-ray spectroscopy a powerful tool for probing local atomic environments, yet whether localized core excitations can report on global collective distortions remains open. Cyclo[18]carbon (C$_{18}$), with its polyynic ground state (D$_\text{9h}$) and cumulenic transition state (D$_\text{18h}$), provides an ideal model to address this long-standing issue in bond-length alternation (BLA) dynamics. Mapping two-dimensional potential energy surfaces by first-principles simulations, we find that core ionization symmetrizes the ground-state double-well potential along the BLA coordinate. Our calculated X-ray spectra reveal remarkable sensitivity to bond-length variations: C1s ionization potentials vary by up to 2.4~eV across the BLA coordinate (1.1–1.4~Å), with a 0.9~eV variation for minima predicted by different functionals, while NEXAFS $\pi^*$ peaks shift by up to 4~eV across the same coordinate. These predicted signatures provide a quantitative spectroscopy–structure dictionary for decoding transient structures in future ultrafast X-ray experiments and monitoring bond-alternation dynamics in real time.
Read the full abstract ▾
All authors Minrui Wei, Zeyu Liu, Sheng-Yu Wang, Jun-Rong Zhang, Lu Zhang, Guoyan Ge, Weijie Hua
Materials Science
Restoring the Surface Magnetic Gap in MnBi$_2$Te$_4$
Ce Bian, Hengxin Tan and Wenhui Duan
A widespread experimental realization of quantized anomalous transport in the intrinsic
magnetic topological
insulator MnBi$_2$Te$_4$ is hindered by its elusive surface
magnetic gap. Uncovering the origin of the gapless states is essential for accessing its topological properties. Here we show that surface defects lower the electrostatic potential, drive topological surface states into subsurface layers, suppress exchange interactions, thereby closing the gap. Tuning the surface electrostatic potential via external electric fields or interfacial fields in van der Waals
heterostructures restores the expected gap and enables control of its
topology. This is confirmed by model calculations and validated in defective MnBi$_2$Te$_4$ films interfaced with polar
insulators, explaining the enhanced
quantum anomalous Hall effect under AlO$_x$ capping observed in recent experiments. Our theory identifies electrostatically driven surface-state delocalization as a competitive origin of gap suppression and proposes displacement-field engineering for robust quantized transport.
Read the full abstract ▾